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Full text of "Text Book Of Mechanical Engineering"

788

Appendix //.

The circular marks are called annular rings, while the radial ones
ate termed medullary rays, and the process of drying tends to
split the wood along the latter, as already shewn. To minimise
this fault the tree should be cut into balks before drying, and the
last should preferably be done naturally and gradually, over
some two or three years, during which time it is protected from
rain, but allowed free air-current Artificial drying or desiccation
produces more splits, or 'shakes' as they are called, and of the
latter, 'cap' shakes follow the rings, and 'star' shakes the rays,'
but the worst shakes are those that twist as they travel along the
log. Speaking generally, timber comes under one or other of two
great divisions—the pine wood, and the non-resinous or leaf wood,
Under the former we have all the softer woods, such as pines,
firs, and spruces; while the latter includes the hard woods, such as
oak, beech, elm. sycamore, ash, mahogany, &c.

CHAPTER   III

P. 74. The Blast Furnace.—Before iron ore is smelted
it is often calcined or heated alone, either in open heaps or in
kilns, one heap of 2000 tons being kept hot for about three weeks.
Some ores, such as the Scotch, require no fuel for tHs purpose,
they themselves containing free carbon. An interesting method
is adopted in Styria, where the ore travels slowly down an inclined
kiln, the fire heat passing in the reverse direction. With the use
of the hot blast,-calcining is not so necessary.

The design of the blast furnace depends largely on the district,
that on p. 73 being from Cleveland, where poor ores and coke
fuel are the rule, and the dimensions large. In Scotland, where
rich ores and coal fuel are used, even a 6o-feet furnace is almost
too high, causing much trouble in keeping up the fuel at the
boshes. A large furnace may have an output of as much as 500
tons per week. The blast, air is heated to about 900° Fahr. by the
waste gases from the top of the furnace, and j:he nature of the
will -depend very, materially on the ore, that for Dowlais